Various systems are known for the detection of alarm conditions. One particular form of such a system is a smoke or fire detecting system of a type generally illustrated in previously issued Tice et al. U.S. Pat. No. 4,916,432, assigned to a common assignee and incorporated herein by reference.
Upon receipt of inputs from one or more of the detectors of the system, a control unit associated with the system is able to make a determination as to whether or not a fire condition is present in one or more regions of interest. A variety of techniques have been used in the past for the purpose of making this determination.
Sensors of smoke such as photoelectric smoke detectors or ionization-type smoke detectors are intended to provide outputs indicative of sensed levels of ambient smoke. Environmental noise, such as dust particles or insects which may enter the respective detector can produce variations in output signals from the sensors which are not in any way correlated with the presence of smoke. These noise outputs can produce false alarms if the sensitivity of the respective detector is high enough. Such false alarms are undesirable.
Photoelectric smoke sensors used for early warning typically use a light source and a light sensitive receiver. The design and placement of the light source, receiver, and baffling are such that no significant light from the source normally reaches the receiver unless smoke or other particles are present in the area of the light beam. If smoke or other particles are present in this area, they will scatter the light photons, and cause some of the light to reach the receiver.
In non-early warning smoke detection systems, the density of smoke required at a sensor to cause an alarm is relatively large compared to the density of dust, fibers and other non-smoke particles normally existing in the environment, therefore these systems are not susceptible to false indications caused by such particles. In early warning smoke detection systems, the signals given by low levels of smoke may be comparable to that given by non-smoke airborne particles in the environment that this type of system is typically used.
In prior art early warning systems, filters were used to remove non-smoke particles in the air present in the smoke sensors. The presence of a filter usually requires that the sensor include a fan or other means to draw air through the filter. The mechanical fans and filters used in prior art detectors are expensive, subject to failure, and require regular maintenance.
Thus there continues to be a need for detectors which can be used in early warning systems without requiring the presence of fans or filters. Preferably minimizing false indications could be accomplished without significantly increasing the expense of such systems while avoiding any need to incorporate additional mechanical components.